Zhi Xiachen

Chemical and isotopic characteristics of gabbroic xenoliths from Hannuoba, China

Nine pieces of gabbroic xenoliths from Hannuoba were examined for their major and trace elements and Nd, Sr and Pb isotopes. The results show that the gabbroic xenoliths are of more mafic basaltic composition. Their abundances show narrow variations in major elements. The trace element contents are highly variable in contrast with those of host basalts and lherzolite xenoliths. The gabbroic xenoliths are rich in Nd (0.51159–0.51249), Sr (0.70491–0.70768) and low in radiogenic Pb(16.283–17.046, 15.191–15.381 and 36.999-37.476), significantly different from basalts and lherzolites in isotopic space. The calculated Nd and Pb model ages are about 3.0–3.5 Ga. The rocks have relatively low equilibriumT(~850°C) andP (0.8–0.9 GPa). They could be interpreted to be the product of upper mantle melting at the boundary between the lower crust and the upper mantle. Their chemical and isotopic variations can be ascribed to different degrees of melting, segregation and long-term evolution.

Re-Os isotopic system and formation age of subcontinental lithosphere mantle

The determination of the formation age of subcontinental Lithosphere Mantle (SCLM) is a widely concerned issue in mantle geochemistry. It is difficult to obtain the formation age of SCLM using lithophile isotopic systems such as Rb-Sr, Sm-Nd, U-Th-Pb, etc., but as siderophile elements, the Re-Os isotopic system provides a powerful tool for that work. Here a comprehensive review on the recent development in Re-Os dating for SCLM has been given.

M1 site splitting due to Next Nearest Neighbor effects and ferric iron in tetrahedral site in clinopyroxene megacrysts

It is well known that in pyroxene structure, there are two metal sites, M1 and M2. Generally speaking, ferrous iron in each of these sites would normally be expected to give rise to a doublet. However, anomalies have been found in the relative areas of the peaks in the room temperature spectra of some clinopyroxene (CPX) when the above assignment is followed. According to the calculation of Next Nearest Neighbor configurations of divalent cations in M1, we found that the four configurations of M1 can be divided into two groups. One group is 3Ca configuration that increases with the content of Ca (p. f. u); the other group is made up of three No-3Ca configurations that decrease with the content of Ca. The two groups contribute to the spectrum structure of M1, so in this study we fit two doublets for ferrous iron in M1.Though there were several reports on Fe3+ in tetrahedral site previously, it was not sure that Fe3+ occupies the T site is a universal fact in CPX, despite of the content of Al. We found that the Fe3+ in the T site fitted by Mössbauer spectroscopy is negatively correlated to the Si content in the T site and positively correlated to the Fe3+ in the T site estimated on the supposition that Fe3+ and Al occupy the T site randomly. If it is true, it is important in the modeling of ion exchange geobarometries and geothermometries.

Trace Element Geochemistry of Tertiary Continental Alkali Basalts from the Liuhe-Yizheng Area, Jiangsu Province, China

Reported in this paper are the chemical compositions and trace element (REE.Ba > Rb, Sr, Nb, Zr, Ni, Cr, V, Ga, Y, Sc, Zn, Cu. etc.) abundances of Tertiary continental alkali basalts from the Liuhe-Yizheng area, Jiangsu Province, China.The olivine basalt, alkali olivine basalt and basanite are all derived from evolved melts which were once affected by different degrees of fractional crystallization of olivine and clinopyroxene (1:2) under high pressures. The initial melts were derived from the garnet lherzolite-type mantle source through low-degree partial melting. The mantle source has been affected by recent mantle-enrichment events (e. g. mantle metasomatism), resulting in incompatible trace element enrichment and long-term depletion of radiogenic isotopic compositions of Sr and Nd.

Hydrogen diffusion in clinopyroxene: dehydration experiments

Hydrogen diffusivity of one clinopyroxene megacryst from Nüshan, eastern China parallel to [001]* crystallographic direction was measured by high-temperature sequential dehydration experiments at 950üC and 850üC, which was (6.5ü1.5)×10-12(m2/s) and (1.8×0.7)×10-12(m2/s) respectively. From these results and the previous published data, it seems that the correlation between hydrogen diffusivity and Fe content suggested by Dyar et al. (1996) possibly does not exist. Clinopyroxene megacrysts from alkali basalts appear to be possible reliable samples for the study of the water evolution and hydrogen isotope compositions of the upper mantle.

Hydrogen isotope compositions of mantle-derived amphibole megacrysts from Qilin, Guangdong and its tectonic significance

Hydrogen isotope compositions of mantle-derived amphibole megacrysts from Qilin, Guangdong Province have been obtained by ion micro-probe. δD and H contents are constant both among different samples and within single sample, demonstrating that their formation condition is very stable. High δD values suggest the presence of a component recycled from crust which is possibly related to the subcluction of Pacific Plate beneath Eurasian Plate in Mesozoic.

Trace element geochemistry of Hannuoba ultramafic inclusion-bearing alkali basalts

Presented in this paper are the trace element abundances of 16 samples of Hannuoba ultramafic inclusion-bearing alkali basalts, which were determined by instrumental neutron activation analysis and X-ray fluorescence spectrometry.The petrogenesis of the alkali basalt suite has been modeled by batch partial melting and Rayleigh fractional crystallization processes. The geochemical characteristics of the mantle source from where alkali basalts were derived are described in terms of variations in trace element abundances of the alkali basalt suite.

Low temperature\(\lg f_{O_2 } - pH\) diagrams of sulfur isotope evolution in an equilibrium hydrothermal system

AbstractAccording to Sakai-Ohmotos theory regarding the evolution of sulfur isotopes in hydrothermal systems, in conjunction of new data on chemical resaction equilibrium constants and equilibrium isotopic fractionation factors as well as on individual ion activity coefficients of aqueous sulfur species, the following lgfo2.-pH diagrams are constructed:a)mole fractions of aqueous sulfur species (Xi),b)stability fields of some minerals in the Fe-S-O system,c)diagram depicting the oxidation-reduction-state ratio for aqueous sulfur species (R′)d)isotopic compositions of sulfur compounds (

Study on Analytical Method for Re-Os Isotopic System in Mafic-Ultramafic Rocks

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